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Ch. 2—Key concepts Correct identification of fossils is the basis for all subsequent interpretations and applications; an understanding of intraspecific.

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Presentation on theme: "Ch. 2—Key concepts Correct identification of fossils is the basis for all subsequent interpretations and applications; an understanding of intraspecific."— Presentation transcript:

1 Ch. 2—Key concepts Correct identification of fossils is the basis for all subsequent interpretations and applications; an understanding of intraspecific variation is necessary for correct identification Ontogenetic variation occurs during an individual’s lifespan Population variation occurs among individuals within a given population Fossils & Evolution Ch. 2

2 Ch. 2—Key terms Ontogeny; ontogenetic variation Population variation
Types of skeletal growth Addition; accretion; molting; modification; combination Isometric vs. allometric growth Principle of similitude Ecophenotypic variation Sexual dimorphism Fossils & Evolution Ch. 2

3 Ontogenetic variation
Ontogeny = the life history of an individual (both embryonic and post-natal) Understanding ontogeny is important because growth stages of an individual may be so different that they are hardly recognizable as the same species Fossils & Evolution Ch. 2

4 Types of skeletal growth
Accretion (enlargement) of existing parts Addition of new parts Molting Modification Combinations (mixed growth strategies) Fossils & Evolution Ch. 2

5 Skeletal growth—Accretion
Accretion = adding new material to an existing shell Allows uninterrupted use of shell and more or less continuous growth Disadvantage is that adult shape is somewhat constrained by juvenile shape Example: bivalve growth Fossils & Evolution Ch. 2

6 Bivalve accretion Fossils & Evolution Ch. 2

7 Skeletal growth—Addition of new parts
Echinoderms may grow simply by adding new plates to their calyx or new columnals to their stalk Example: crinoid stalk Large columnals added just beneath calyx Smaller columnals added between larger ones Alternation of different sizes allows increased flexibility Fossils & Evolution Ch. 2

8 Crinoid stalk (addition)
Fossils & Evolution Ch. 2

9 Skeletal growth—Molting
Molting = periodic shedding of an exoskeleton followed by growth of a new, larger one Advantage: Shape of adult organism not constrained by shape of juvenile stages Disadvantages are (1) vulnerable period during the molt itself; (2) significant metabolic cost of repeatedly replacing entire skeleton Example: trilobites Fossils & Evolution Ch. 2

10 Trilobite molting Instars = growth stages between molts
Fossils & Evolution Ch. 2

11 Molting (cont.) Molting produces growth in instars
a series of discrete episodes (not continuous)—Instars from different growth stages form distinct morphologic clusters instars Fossils & Evolution Ch. 2

12 Skeletal growth—Modification
Modification = process of replacement and re-formation of skeletal material, allowing size increase as well as changes in shape and structure Skeletal form of adult is not strongly constrained by skeletal form of juvenile No vulnerable stage (as in molting) Example: vertebrate bones Fossils & Evolution Ch. 2

13 Skeletal growth—Mixed strategies
Some organisms employ combinations of growth strategies Example: coiled cephalopod grows by accretion along leading edge of shell and also by periodic addition of septa Fossils & Evolution Ch. 2

14 Combined growth strategy (coiled cephalopod)
continuous accretion of new material along leading edge of shell periodic addition of new septa Fossils & Evolution Ch. 2

15 Recognizing and describing ontogenetic change
Biologists can directly observe ontogenetic change, but paleontologists cannot Two main approaches to studying ontogenetic changes in fossil material: Growth series of specimens representing different developmental stages (as in successive trilobite instars) Adult specimens whose development is recorded by growth lines or newly added parts (as in bivalve example) Fossils & Evolution Ch. 2

16 Recognizing and describing ontogenetic change
Approach depends on the kinds of fossils being studied: Cannot use adult specimens to study ontogeny in animals that grow through molting or modification Fossils & Evolution Ch. 2

17 Example 1: Brachiopod ontogeny
Length and width measurements performed on large (~75) population of specimens of all sizes Plot of length vs. width suggests change in shape during growth Small individuals are wider than long Large individual are longer than wide Fossils & Evolution Ch. 2

18 Brachiopod example: Length vs. width
Growth Series: scatter of data points suggests change in shape during growth Fossils & Evolution Ch. 2

19 Example: Brachiopod ontogeny
A more definitive understanding of brachiopod ontogeny can be achieved by plotting growth curves for individual specimens (by measuring along growth lines) Fossils & Evolution Ch. 2

20 Brachiopod example: Length vs. width
Individual ontogeny: growth curves for single specimens confirm change in shape, AND allow estimate of variation among individuals Fossils & Evolution Ch. 2

21 Types of growth Isometric = no change in shape during ontogeny (ratio between parts does not change as size increases) Relatively uncommon Anisometric (allometric) = change in shape during ontogeny (ratio between parts changes as size increases) Relatively common Fossils & Evolution Ch. 2

22 Types of growth (cont.) Consider two body parts, X and Y
As organism grows, relationship between X and Y is given as: In isometric growth, a = 1 (linear equation) In anisometric growth, a = 1 (curve) Y = bXa Fossils & Evolution Ch. 2

23 Isometric growth Fossils & Evolution Ch. 2

24 Anisometric growth Fossils & Evolution Ch. 2

25 Why is anisometric growth common?
Anisometric growth is necessary in most organisms because volume (body mass) increases as the cube of linear size increase Example: bone strength is proportional to cross-sectional area of bone As linear dimensions of bone doubles, cross-sectional area is squared, but body mass is cubed Body weight increases faster than relative strength of supporting bones This scaling inequality is “principle of similitude” Fossils & Evolution Ch. 2

26 “Principle of similitude”
10 20 2 2 Cross-sectional area = 16 Volume = 320 Cross-sectional area = 4 Volume = 40 4 4 Fossils & Evolution Ch. 2

27 Anisometry of pelycosaur femurs (note different shapes as well as different sizes)
Fossils & Evolution Ch. 2 decreasing size of animal

28 Population variation Variation among individuals within a population is called population variation Sources of population variation are: Genetic differences among individuals Ecophenotypic variation Sexual dimorphism Taphonomic effects Fossils & Evolution Ch. 2

29 Populations Biologic definition of population = “a group of individuals of the same species living close enough together that each individual of a given sex has a chance of mating with an individual of the other sex” “breeding population” Populations are characterized by a single gene pool Gene flow occurs when two or more populations interbreed Fossils & Evolution Ch. 2

30 Genetic variation: Alternation of generations in forams
“megalospheric” (asexually produced) “microspheric” (sexually produced) Fossils & Evolution Ch. 2

31 Ecophenotypic variation
Variation among individuals as a consequence of differences in their environments: Nutrition Exposure to sunlight (plants; animals with phtotsynthesizing symbionts) Space (crowding) Environmental stability Fossils & Evolution Ch. 2

32 Sexual dimorphism in ammonoids
dimorphic pair dimorphic pair Fossils & Evolution Ch. 2

33 Fossil populations Not as easy to work with as biologic (living) populations Sources of difficulty Sedimentary mixing (reworking; bioturbation) Time-averaging; loss of temporal resolution Preservation bias Distortion Dissolution (reduces observable variation) Post-mortem sorting Fossils & Evolution Ch. 2

34 Structural distortion of bivalve shapes
direction of rock cleavage undeformed shape Fossils & Evolution Ch. 2

35 Effects of selective post-mortem transport
Fossils & Evolution Ch. 2

36 Fossil populations (cont.)
Additional example of population “biasing” by selective transport Devonian brachiopods Leptocoelia (879 pedicle; 893 brachial) Platyorthis (561 pedicle; 548 brachial) Leptostrophia (378 pedicle; 35 brachial) untransported, or not selectively transported selectively transported Fossils & Evolution Ch. 2

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